1. Statement of the Technical Field
The present disclosure relates to optical modulation devices, and more particularly to resonator-based optical modulators.
2. Description of the Related Art
Advanced optical modulation formats could offer significant advantages for optical communications. For example, Quadrature Phase-Shift Keying (“QPSK”) provides higher spectral efficiency, better tolerance to fiber nonlinearly and chromatic dispersion, and enhanced receiver sensitivity compared to on-off keying. Traditional Lithium Niobate (“LiNbO3”) modulators can be used for such modulation. However, LiNbO3 modulators are relatively large in size. For a general M-ary modulation format that requires a large number of optical modulator components along with their driving signal circuitries, the overall size of the entire modulator is rather cumbersome. Recent breakthroughs in silicon photonics, particularly silicon based optical modulators, have fundamentally changed the landscape of modulator technology. Notably, micro-resonator based silicon modulators constitute an ideal candidate for optical modulation due to their compact size, low power consumption, and ease of monolithic integration with driving circuitries on the same silicon chip. Most research on silicon micro-ring modulators employed intensity modulation in binary formats. Recently, micro-ring resonator based modulators for differential binary phase-shift-keying and differential QPSK have been proposed, and satisfactory performances have been predicted. Another work employed the anti-crossing between paired amplitude and phase resonators and demonstrated enhanced sensitivity to the input drive signal. A high-Q micro-ring quadrature modulator incorporating dual 2×2 Mach-Zehnder interferometers has also been recently proposed with beneficial performance.